1. Field of the Invention
This invention generally relates to an electrophotographic photoreceptor suitable for flash fixing. In addition, the present invention relates to an image forming device that employ the same, such as laser printers, electrostatic copying machines, plain paper facsimile devices, and multi-function devices which combine these functions.
2. Background Information
A conventional image forming device electrostatically charges the surface of an electrophotographic photoreceptor, exposes an original document having an image thereon, and forms an electrostatic latent image on the surface of the photoreceptor that corresponds to the image on the original document. After developing the electrostatic latent image with toner, the image forming device transfers the toner formed on the photoreceptor to a recording medium such as paper. The recording medium is then separated from the photoreceptor, and an image is formed thereon by fixing the toner. After the toner is transferred to the recording medium, any remaining electric charge on the photoreceptor is removed, and an electrostatic charge is again placed on the photoreceptor in order to produce another image. One means for fixing the toner image that is employed by this type of image forming device is a flash fixing means which uses a flash lamp.
In the flash fixing means, the toner absorbs the radiant heat or light energy from flash light source, is heated and melted thereby, and then fixed to a recording medium. When this occurs, the absorption of the light energy is limited to the toner only; the recording medium itself absorbs almost no light energy. Thus, if a flash fixing means is employed, there is little damage to the recording medium from the heat used during fixing, because of the small degree to which the temperature of the image forming device is increased due to this heat.
However, in conventional image forming devices, the light that radiates from the flash lamp (the flash light) leaks from the flash fixing means and reaches the photoreceptor. Normally, the flash fixing means and the recording medium do not come into contact with each other, and have a fixed gap formed between each other. This configuration allows a toner image to be fixed on the recording medium by radiating the flash light from a predetermined distance thereabove, and also serves to prevent any unfixed portion of the toner image on the recording medium from becoming smeared. In addition, because fixing is performed after the transfer process, the flash fixing means is disposed downstream of the transfer device in the transport direction, and is disposed comparatively close to the photoreceptor.
Because of this configuration, flash light leakage from the flash fixing means is inevitable, and thus leaked light radiates onto the photoreceptor after the transfer process has been completed. When this occurs, the portion of the photoreceptor on which the leaked light is radiated onto will generate a positive or negative electrical charge by means of the charge generating agent therein, and will neutralize the electrical charge on the surface of the photoreceptor that was placed thereon after the transfer process. In other words, an electrical charge that has a polarity that is the reverse of the electrical charge on the surface of the photoreceptor will neutralize the same, and because of this, an electrical charge with the same polarity will move toward the support substrate of the photoreceptor. When this occurs, the surface electric potential of the photoreceptor will be reduced to a certain extent because a reverse bias electrical potential is placed thereon in the transfer process. When light leaked from the flash fixing means is radiated onto the photoreceptor in this state, the electric potential on the portion of the photoreceptor on which the light was leaked will immediately drop, regardless of whether it is an exposed portion or unexposed portion.
Further, when the leaked light produces an electric charge in the photosensitive layer of the photoreceptor, the electric charge will remain therein because there is no electric charge on the surface of the photoreceptor that will neutralize it. The electric charge in the photosensitive layer will continue to remain there even after the electric charge on the surface of the photoreceptor is removed after the transfer process. A uniform electrostatic charge is placed on the surface of the photoreceptor in the electrostatic charging process. However, the electric charge on the portion of the photoreceptor on which the leaked light has been radiated will be neutralized by the electric charge remaining in the photosensitive layer. Because of this, the surface electric potential of this portion of the photoreceptor after the electrostatic charging process (and before the exposure process) will be lower than other portions thereof. In addition, the portion of the photoreceptor on which the leaked light has been radiated will not be properly developed, and thus images therefrom will be uneven.
Furthermore, when leaked light is repeatedly radiated onto the photoreceptor, the photoreceptor will become increasingly degraded, and its sensitivity and its ability to be electrostatically charged will decrease. In addition, the density of the image formed on the recording medium will decrease, and the image thereon will become blurred. The degradation in the photoreceptor is thought to be primarily due to an increase in the number of molecules of the charge generating agent that have lost their ability to function as photoconductors after repeatedly generating and discharging an electric charge when optically illuminated.
It is thought that this problem can be prevented from occurring by controlling the amount of light that leaks from the flash fixing means. However, this is difficult from a structural point of view because the area around the flash fixing means cannot be sealed off. On the other hand, reducing the amount of flash light has also been considered. This will result in a reduction in the amount of leaked light, but the fixity of the toner image will worsen. Thus, controlling the amount of light that leaks from the flash fixing means is in actuality quite difficult.
In addition, Japanese Published Patent Application Nos. H06-167906 and H06-236133 disclose using leaked light to actively remove the electric charge from the photoreceptor. In these methods, the transportation of the recording medium to the fixing means must be timed, and the leaked light must be radiated onto the entire electrostatic latent image on the photoreceptor. Because of this, not only is the placement of the fixing means and photoreceptor limited to certain positions, but the photoreceptor can only be one which forms one image per one or less rotation thereof (e.g., a drum shaped photoreceptor which has a large diameter and thus rotates less), thereby making it difficult to reduce the size of the image forming device. In order to use a photoreceptor that requires more than one rotation thereof to form one image, the flash lamp must be illuminated both during and after the fixing process, and thus will increase the cost of operating the image forming device.